Robin Williams, the beloved comic who committed suicide on Monday, was battling the early stages of Parkinson's disease, it has been revealed by his widow, Susan Schneider.

In a statement sent to the news media on Thursday, Schneider explains that the star's sobriety was "intact" at the time of his death, calling him "brave" as he battled depression, anxiety and early stages of the disease.

His diagnosis, Schneider adds, was something Williams was "not yet ready to share publicly." The actor had struggled for decades with substance abuse and mental-health issues.

She asked that her following statement run in its entirety.

"Robin spent so much of his life helping others. Whether he was entertaining millions on stage, film or television, our troops on the frontlines, or comforting a sick child — Robin wanted us to laugh and to feel less afraid."

"Since his passing, all of us who loved Robin have found some solace in the tremendous outpouring of affection and admiration for him from the millions of people whose lives he touched. His greatest legacy, besides his three children, is the joy and happiness he offered to others, particularly to those fighting personal battles."

"Robin's sobriety was intact and he was brave as he struggled with his own battles of depression, anxiety as well as early stages of Parkinson's Disease, which he was not yet ready to share publicly."

It is our hope in the wake of Robin's tragic passing, that others will find the strength to seek the care and support they need to treat whatever battles they are facing so they may feel less afraid."

Actor Michael J. Fox tweeted Thursday evening that he was "stunned" by the news that Williams had the disease. "Pretty sure his support for our (foundation) predated his diagnosis. A true friend; I wish him peace." Fox was diagnosed with Parkinson's in 1991.

Stunned to learn Robin had PD. Pretty sure his support for our Fdn predated his diagnosis. A true friend; I wish him peace.

The brain disease, which often leads to depression, can elevate someone's risk of suicide, says Jeffrey Lieberman, Professor and Chairman of Psychiatry at Columbia University College of Physicians and Surgeons and Director, New York State Psychiatric Institute.

More than 60% of people with Parkinson's in a 2012 study reported depression, and 18% reported severe, major depression.

Williams was already at higher than average risk for suicide because of his history of depression and substance abuse, as well as his demographic group. Older white men, particularly those who have had heart surgery, as he had, are at higher risk. Parkinson's "is another risk factor both psychologically and physiologically," Lieberman says.

Depression appears to develop not because people are sad that they have a disability, but because of damage caused by Parkinson's to the brain, says neurologist Irene Richard, an adviser to the Michael J. Fox Foundation and a professor at the University of Rochester Medical Center.

The first signs of Parkinson's often include a tremor, a slight shaking of the finger, hand, chin, lip or leg. Other early signs, according to the National Parkinson's Foundation, include shrinking handwriting, reduced sense of smell, sleep problems, awkward movements, or a stiff or frozen facial expression.

More people in America are living with Parkinson's disease than those diagnosed with multiple sclerosis, muscular dystrophy and Lou Gehrig's disease combined. There are no tests to diagnose the condition and the exact cause is still unknown. A glance at the number affected and factors involved in living with the disease: